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1.4 Sea-floor spreading

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Vocabulary

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mid-ocean ridge- the longest chain of mountains in the world
sonar- sonar is a device that bounces sounds waves off underwater objects and records the echoes of the waves
sea-floor spreading- the process that continually adds new material to the ocean floor
deep ocean trenches- when the ocean floor plunges into deep underwater canyuns
subduction- the process by which the ocean floor sinks beneath a deep ocean trenches and back into the mantle

Outline

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Introduction

Mapping the Mid-Ocean Ridge

• use sonar devices

Evidence for Sea-Floor Spreading

• Evidence from Molten Material
  • The presence of rocks showed that molten material has erupted again and again from cracks along the central valley of themid-ocean ridge.
• Evidence from Magnetic Strips
  • Long ago the poles have reversed themselves due to sea-floor spreading.
• Evidence from Drilling Samples
  • They found that the farther away from the ridge the samples were taken, the older the rocks were. The youngest rocks were always in the the center of the ridges.

Subduction at Deep Ocean Trenches

• Subduction in the Pacific Ocean
  • The Pacific Ocean is shrinking because the deep-ocean trenches swallow more oceanic crust than the mid-ocean ridge can produce.
• Subduction in the Atlantic Ocean
  • The Atlantic Ocean is expanding because it has has few short trences. So the spreading ocean floor is going nowhere.